Transcript

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Suvarna Patil et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 2), February 2014, pp.21-25
RESEARCH ARTICLE
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OPEN ACCESS
Delay Tolerant Networks – Survey Paper
SuvarnaPatil*,Geetha R. Chillerge**
*(ME in Computer Engineering (SEM IV), MMCOE, Pune, India)
**(Assistant Professor, Department of Computer Engg., MMCOE, Pune, India)
ABSTRACT
This paper provides an introduction to Delay Tolerant Networks (DTN) and would touch upon some
basic features. Continuous connectivity is difficult in today’s wireless world. The data preservation
and security in challenged and intermittent network, is of paramount importance. In this paper, we
will see how DTN provides an effective alternative. This will also try to explain basic architecture of
DTN and routing techniques that can be incorporated for effective data forwarding. Security of data
becomesimportant in disrupted networks; this paper would also discuss security concerns with DTNs.
This paper also discusses possible applications and areas where DTN can be effectively used.
Keywords: Bundle Protocol, Contact Patterns, DTN, Flooding, Forwarding, Routing
I. INTRODUCTION
Internet has been successfully connecting
communicating devices worldwide today. TCP/IP
protocol suite plays most important role in achieving
this effectively. Every device on the countless subnetworks that comprise the Internet makes use of this
protocol for data transfers from source to destination
with the minimal possible delay and high reliability.
End to end data transfer is the basic principle on
which TCP/IP is based on. However assumptions of
internet cannot hold in many regions. If there
instances where end-to-end connectivity is broken or
intermittent, then TCP/IP may not work correctly and
reliably, in many cases it can completely fail to
transfer data from source to destination.
Basically internet (TCP/IP protocol) works
based on certain assumptions:
- End-to-end path between source and destination
exists forthe duration of a communication
session
- Retransmissions based on timely and stable
feedback from data receivers is an effective
means for repairing errors
- End-to-end loss is relatively small
- All routers and end stations support the TCP/IP
protocols
- Applications
need
not
worry
about
communication performance
- Endpoint-based security mechanisms are
sufficient for meeting most security concerns
- Packet switching is the most appropriate
abstraction for interoperability and performance
- selecting a single route between sender and
receiver is sufficient for achieving acceptable
communication performance
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Such networks suffer from frequent temporary
partitions which can be termed as Intermittently
Connected Networks (ICNs). This problem occurs
mainly in remote areas, or villages that lack basic
infrastructure to support internet.
Due to suchcircumstances, a newer network
has evolved which is independent of end-to-end
connectivitybetween nodes. This network is called as
Delay Tolerant Networks (DTN).Delay Tolerant
Networking (DTN) is an approach to computer
network architecturethat aims to address the technical
issues in heterogeneous networks that experience
lack ofcontinuous network connectivity. Delay
Tolerant Networks (DTNs) enable data transferwhen
mobile nodes are only intermittently connected. Since
the connectivity is not expected to be consistent in
DTN, it employs what is called a store-carry-andforward routing mechanism. In this, the intermediate
mobile nodes carry data packets when they receive it
and forward it to the next node as and when contact is
established. As DTN depends on mobile nodes to
carry data, the performance of routing the data solely
depends on whether the nodes come in contact with
each other or not.
What is DTN
Delay tolerant networks (DTNs) represent a
class of wireless systems that virtually need
minimum to none infrastructure and would support
the functionality of networks experiencing frequent
and long lasting partitions. DTNs are intended to deal
with scenarios involving heterogeneity of standards,
intermittent connectivity between adjacent nodes,
lack of contemporaneous end-to-end links and
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Suvarna Patil et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 2), February 2014, pp.21-25
exceptionally high delays and error-rates. Also the
mobile nodes available in challenged environments
can be extremely limited in their resources; such as
CPU processing power, memory and network
capacity. As DTNs are expected to tackle such dared
environments, they are usually intended to achieve
interoperability and eventual connectivity to a range
of complex applications that include:
- Wireless sensor networks (WSNs) deployed in
wildlife tracking or in extreme regions (e.g.
volcanic and underwater areas).
- Mobile Ad-Hoc networks connecting remote and
rural communities via GPSs, cellular devices and
portable storages.
- Exotic Media Networks (EMNs) interconnecting
extra-terrestrial nodes such as satellites and deep
space probes in Inter-Planetary Networks (IPNs).
Each of the potential field of applications
mentioned above is intended to operate under
stressful circumstances and in environments that are
considered to be challenging for ordinary wireless
nodes within a traditional network settings.
DTN architecture represents an attempt to
extend the reach of networks. It promises to enable
communication between instances of such challenged
networks and to act as an integral platform between
instances that originally adopt heterogeneous or
inconsistent standards, even if they exist in territories
lacking a proper communication infrastructure. The
main purpose of the DTN approach is to provide a
means for message delivery in such challenged
settings.
II. ARCHITECTURE OF DELAY
TOLERANT NETWORK
The architecture of DTN is designed in such
a way that it counters most of the assumptions and
conditions that traditional TCP/IP based networks are
based on. DTN architecture is based on following
design principles:
- Use variable-length (possibly long) messages
(not streams or limited-sized packets) as the
communication abstraction to help enhance the
ability of the network to make good
scheduling/path selection decisions when
possible.
- Use a naming syntax that supports a wide range
of naming and addressing conventions to
enhance interoperability.
- Use storage within the network to support storeand-forward operation over multiple paths, and
over potentially long timescales (i.e., to support
operation in environments where many and/or no
end-to-end paths may ever exist); do not require
end-to-end reliability.
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Provide security mechanisms that protect the
infrastructure from unauthorized use by
discarding traffic as quickly as possible.
Provide coarse-grained classes of service,
delivery options, and a way to express the useful
lifetime of data to allow the network tobetter
deliver data in serving the needs of applications.
Fig 1: DTN Architecture Principles
a.
Concept of Bundle Protocol
A Delay Tolerant Network can be
considered as an overlay on the existing regional
networks. This overlay is called as the bundle layer.
This layer is intended to function above the existing
protocol layers and provide the function of a gateway
when two nodes come in contact with each other. The
main advantage of this kind of protocol is flexibility.
It can be easily linked with the already existing
TCP/IP protocol networks or can be used to link two
or more networks together. The position of the
bundle layer can be seen in the following fig. 2.
Fig 2: Bundle Layer
Bundles are also called as messages. The
transfer of data from one node to another can be
made reliable by storing and forwarding entire
bundles between nodes. The bundles comprise of
three things, source node’s user-data, control
information (e.g., source node ID, destination node
ID, TTL etc.), a bundle header. Besides Bundle
transfer, custody transfer is also done. The custodian
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Suvarna Patil et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 2), February 2014, pp.21-25
node for a bundle keeps the message until it is
successfully transferred to the next node and it takes
the custody for that message or until the TTL of the
message expires.
b.
Store and Forward Technique
Delay Tolerant Networks have overcome the
problems associated with the conventional protocols
in terms of lack of connectivity, irregular delays,
asymmetric bidirectional data rates etc. using the
concept of store and forward. The method of store
and forward is very analogous to the real life postal
service. Every letter has to pass through a set of post
offices, here it is processed and forwarded, before
reaching the destination. Here the complete message
or a chunk of it is transferred and stored in nodes
successively until it reaches the destination. The
following figure (fig. 3), gives a rough graphical
representation of how a message is propagated
through a network.
d.
e.
Fig 3: Store and Forward Approach
c.
Types of Contacts
Store and forward mechanism solely
depends on whether or how the nodes make contact
with each other. Contacts typically fall into one of
several categories, based largely on the predictability
of their performance characteristics and whether
some action is required to bring them into existence.
Following are major types of contacts that can be
defined:
a. Persistent Contacts
Persistent contacts are always available (i.e., no
connection-initiation action is required to
instantiate a persistent contact). An 'always-on'
Internet connection such as a DSL or Cable
Modem connection would be a representative of
this class.
b. On-Demand Contacts
On-Demand contacts require some action in
order to instantiate, but then function as
persistent contacts until terminated. A dial-up
connection is an example of an On-Demand
contact (at least, from the viewpoint of the dialer;
it may be viewed as an Opportunistic Contact,
below, from the viewpoint of the dial-up service
provider).
c. Intermittent - Scheduled Contacts
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A scheduled contact is an agreement to establish
a contact at a particular time, for a particular
duration. An example of a scheduled contact is a
link with a low-earth orbiting satellite. A node's
list of contacts with the satellite can be
constructed from the satellite's schedule of view
times, capacities, and latencies. Note that for
networks with substantial delays, the notion of
the "particular time" is delay-dependent. For
example,
a single scheduled contact between
Earth and Mars would not be at the same instant
in each location, but would instead be offset by
the (non-negligible) propagation delay.
Intermittent - Opportunistic Contacts
Opportunistic contacts are not scheduled, but
rather present themselves unexpectedly. For
example, an unscheduled aircraft flying overhead
and beaconing, advertising its availability for
communication, would present an opportunistic
contact. Another type of opportunistic contact
might be via an infrared or Bluetooth
communication link between a personal digital
assistant (PDA) and a kiosk in an airport
concourse. The opportunistic contact begins as
the PDA is brought near the kiosk, lasting an
undetermined amount of time (i.e., until the link
is lost or terminated).
Intermittent - Predicted Contacts
Predicted contacts are based on no fixed
schedule, but rather are predictions of likely
contact times and durations based on a history of
previously observed contacts or some other
information. Given a great enough confidence in
a predicted contact, routes maybe chosen based
on this information.
III. ROUTING TECHNIQUES
Connectivity of nodes in DTN is always
going to be intermittent, no two nodes can be always
connected. So end-to-end connectivity never exists.
DTN uses store-carry-forward mechanism to transfer
the data packets across source and destination nodes.
To make this possible the intermediate nodes take
custody of data packets or bundles and forward it
when the opportunity arises.
So for effective data transfer and
performance of a DTN, a route must be optimally and
strategically formed between the nodes. There are
several strategies that are employed for routing, a
brief overview of these strategies is presented here.
The routing strategies are categorized as Flooding
Strategies and Forwarding Strategies.
a.
Flooding Strategies
In a flooding strategy, a message is
duplicated into several messages and the copies are
delivered to a set of nodes called relay nodes. Relay
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Suvarna Patil et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 2), February 2014, pp.21-25
nodes stores the messages until they come in contact
with destination node. As soon as the contact is made
during "contact" phase, the messages are delivered by
the relay nodes. Many of the strategies are workout
by the researchers before this DTN get popular.
These strategies are studied in the context of, mobile
adhoc network where random mobility is good
chance of bringing the source in contact with
destination. Message replication is then used to
increase the probability that the message is
successfully delivered to the intended node. These
protocols do not require any prior global or local
knowledge about network.
b.
Forwarding Strategies
In forwarding strategy knowledge about the
network is used to calculate a best path of nodes
between source and destination. A message is then
forwarded from node to node along this path. So
there is not duplication of data in these strategies.
However the protocols designed for such strategies
need to have the knowledge about network topology
so as to find optimal path without replicating the data
packets.
IV. SECURITY
Delay-tolerant networks can face some
serious resource crunches in some situations, this
demands some form of authentication and access
control to the network in such situations. IT should
not be possible for an un-authorized user to flood the
network with traffic. Access should be controlled
strictly for such users. In most of the cases it is also
not acceptable for unauthorized traffic to be
forwarded over certain network links at all. This is
critical especially for exotic, mission-critical links.
Considering these potential security issues,
several goals have been established for designing the
security of a typical DTN architecture:
Immediately
prevent
unauthorized
applications from having their data carried through or
stored in the DTN.
Strictly avoid unauthorized applications
from declaring control over the DTN infrastructure.
Do not allow to send bundles at nonpermissible rate or class of service, even though the
application is authorized for transfers.
Bundles may be damaged or tampered with
in transit, such should be identified and discarded.
There should be a mechanism to detect
compromised entities, and when they are found their
authorization should be revoked immediately.
Most of the existing authentication and
access control mechanisms or protocols are designed
for operation in non-delayed and always connected
networks, and may not necessarily function
efficiently when applied to DTNs. Especially
dynamic updates to ACLs (Access Control Lists) and
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blacklisting or revoking credentials for a particular
compromised node can be difficult. Also, methods
that require frequent access to centralized servers to
complete an authentication or authorization
transaction are not very suited for DTNs. As a
consequence of these problems, there is a delay in
the commencement of communication
detecting and recovering from system
compromise
Completing
transactions
due
to
inappropriate access control or authentication
settings.
Due to these challenges, to meet the security
requirements, DTN architecture implements a
standard but optionally deployed security architecture
that uses hop-by-hop and end-to-end authentication
and integrity mechanisms. The purpose of using both
approaches is to be able to handle access control for
data forwarding and storage separately from
application-layer data integrity. For a principal such
as a user (of which there may be many), the end-toend mechanism provides effective authentication.
Whereas the hop-by-hop mechanism is intended to
authenticate DTN nodes as genuine transceivers of
bundles for each-other. It is plausible to construct a
DTN in which not all of the nodes participate in the
security mechanisms, but rather a subset of nodes
take part, which results in a secure DTN overlay
existing
on top of an apparently insecure DTN
overlay.
DTN nodes are expected to discard traffic as
soon as practically possible if the authentication or
access control fails, this will help satisfy above goals
of secure network. This approach has the associated
benefit of making denial-of-service attacks
considerably harder to stand as compared with
conventional Internet routers. However, the obvious
cost for this capability is potentially larger
computation and credential storage overhead required
at DTN nodes.
V. APPLICATIONS OF DTN
The store-carry-forward architecture of DTN
was originally an idea that was designed to fulfil the
requirements of the Interplanetary Internet (IPN).
DTN is just a simplified form of this network
architecture. So, the primary objectives of DTN were
to survive complex and challenging network
environments and failures in hardware as well as
software e.g. protocol failures. Even if the DTN was
originally designed to be more of tactical purpose. It
may and will have far greater applications in real
day-to-day world. Some of such applications are
listed here:
Space Agencies: International Space Station
communication (currently operational for research),
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Suvarna Patil et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 2), February 2014, pp.21-25
interplanetary communication, future space-debris
monitoring.
Military and Intelligence: Mobile ad-hoc
networks (MANETs) for wireless communication
and monitoring, cargo tracking, search and rescue
communication, unmanned aerial vehicle (UAV)
communication and control.
Commercial: Cargo and vehicle tracking (by
road, rail, sea, and air), in-store and in-warehouse
asset tracking, data transactions (e.g., financial,
reservations),
agricultural
crop
monitoring,
processing-plant monitoring, communication in
underground mines.
Public Service and Safety: Security and
disaster communication, search and rescue
communication, humanitarian relief monitoring,
smart-city event-response, smart transportation
networks, smart electric-power networks, global
airport-traffic
control,
infrastructure-integrity
monitoring, unmanned aerial vehicle (UAV)
communication and control, remote learning.
Personal Use: Personal monitoring and
communication in wilderness and urban areas, fireand-forget text messaging.
Environmental
Monitoring:
Animal
migration, soil properties and stability, atmospheric
and oceanographic conditions, seismological events.
Engineering and Scientific Research:
Network subject-matter experts, academic research
by faculty and students.
However the biggest challenge for DTN
remains to be effective security implementation. As
the data transfers are intermittent and data may end
up to be with relay node for longer durations, the
security of data must be carefully designed and
should be very strict.
VII. ACKNOWLEDGEMENTS
I would like to express my sincere thanks to
my academic guide Geetha Chillarge, for
encouraging and helping me throughout this work.
I would also like to mention V. Cerf,S.
Burleigh, A. Hooke, L. Torgerson and others who
authored “Delay-Tolerant Networking Architecture”,
a paper that helped me a lot to understand DTN and
is referred here. Also thanks to all other authors
whose work I have referred in this survey.
REFERENCES
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[2]
VI. CONCLUSION
Delay Tolerant Networks will form one of
the most important facets of modern day networking,
given the necessity of connectivity. The traditional
TCP/IP protocol suite is not suited to these modern
day mobile networks as it relies on end-to-end
connectivity being always present and a very low
error rate.
Bundle protocol in DTN offers a viable
solution for challenged or intermittent networks and
can be easily plugged into existing TCP/IP network.
This uses store-carry-forward mechanism to transfer
the data between intermittently connected nodes.
DTN can have most common application in
modern social networks where social contacts are
connected intermittently, however it has capabilities
that can extend it to be applicable to much challenged
network conditions such as used by space agencies or
military and intelligence agencies.
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